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DAC with ocean storage icon

DAC with ocean storage

From £679/tonne · Questions? Talk to sales

Overview

This process uses seawater electrolysis to capture and convert atmospheric CO₂ into two forms: carbonate solids (which can be used in construction) and dissolved bicarbonate ions that are permanently stored in the ocean.

Our supplier’s approach is unique because it does this onshore in a closed system, making it easier to measure carbon removal.

Using electrolysis, water is split into two streams. One stream is acidic, and the other is alkaline. The alkalinity leads to the formation of solid calcium carbonate and magnesium hydroxide. The calcium carbonate formation removes CO₂ from the water and forms solids (limestone) that can be stored or used in construction. In the second step, the alkaline stream containing the magnesium hydroxide solids is contacted with air, which converts atmospheric CO₂ into bicarbonate ions.

The electrolysis process strips the seawater of its magnesium and calcium ions. In order to maintain the original chemistry before the  water is re-released into the ocean, the cations are replenished. This is done by running the acidic stream over silicate rocks, which releases magnesium and calcium ions back into the water. Then the two streams are combined again under rigorous monitoring to ensure the water leaving the plant has a similar composition to that that came in, and is compliant with permitting regulations.

The science

Vast availability of Ca²⁺ and Mg²⁺ ions in the oceans means that CO₂ dissolved in seawater should be able to readily precipitate into Mg-carbonates and Ca-carbonates. However, kinetic constraints mean this is largely prohibited. Certain organisms have evolved to overcome these constraints using biological processes.

For example, to produce calcium carbonate shells. However, abiotic precipitation is very rare. In Equatic’s process, electrolysis is used to create high pH conditions in order to overcome these constraints. This leads to the instantaneous precipitation of calcium carbonate (CaCO₃) and magnesium hydroxide (Mg(OH)₂).

The CaCO₃ precipitation represents the removal and highly permanent storage of CO₂ previously dissolved in seawater. However, the majority of the CDR in this process comes from a second stage in which the alkaline stream containing the precipitated Mg(OH)₂ and remaining Ca²⁺ ions is aerated. This aeration process leads to the absorption of CO₂ which causes the progressive dissolution of Mg(OH)₂. The result is the immobilization of this atmospherically derived CO₂ in the form of bicarbonate ions (HCO₃⁻) and further CaCO₃ precipitation.

Dissolved oceanic CO₂ concentrations are highly dependent on the pH of the water. This is what helps the absorption of CO₂ by the alkaline stream during the aeration process. However, it also highlights the importance of neutralising the acidic stream. Silicate rocks are used to both neutralise this acidic stream and reintroduce Ca²⁺ and Mg²⁺ ions to the water prior to the effluent being released to the ocean. This ensures no pH related degassing will occur and no major changes to ocean chemistry occur during the process.

Both the bicarbonate and calcium carbonate production immobilizes CO₂ for over 10,000 years. There is an incredibly low risk of reversal making this one of the most permanent methods of CDR available.

Research papers 3

Supercritical‘s view

The ocean is the world’s largest natural carbon sink and oceanic carbon sequestration is pivotal for large-scale CDR.

We love this project because it is a scalable, oceanic initiative with a robust measurement, reporting and verification (MRV) blueprint. While similar to conventional DAC and ocean-alkalinity enhancement, it offers a unique approach to CDR.

By operating in a closed system, all of the inputs, outputs and processes can be monitored as they happen. The co-production of green hydrogen only adds to this great project by offering an alternative way of reducing the energy requirements of the project.

Bojana Bajzelj

Head of Climate

Our suppliers

Spun out of UCLA’s institute for Carbon Management, Equatic is a really exciting company with grand ambitions for gigatonne scale carbon removal.

With two sites operating, a data-led approach and a great team our partnership allows us to support a company that is helping to scale carbon removal with integrity and credibility.

Project locations 2

  • Map of California, USA
    California, USA
  • Map of Singapore
    Singapore

Verifications

Our climate team ensures only high-quality carbon removal makes it onto the Supercritical marketplace.

Each Project has completed our rigorous 8-point vetting process, with fewer than 6% passing.

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The Supercritical marketplace only features projects that:

  • Remove carbon

    We do not deal with carbon avoidance or carbon reduction offsets

  • Are net negative

    A project’s removal must be net of lifecycle emissions

  • Are Measured, Reported and Verified

    Projects must have MRV and unique retirement on an established registry

  • Are additional

    The removal must not have occurred in the absence of a market for offset credits

  • Do no significant harm

    Projects must cause no ecological, economic or social harm

  • Have clear permanence

    Permanence must be well established, and be conservative

  • Have co-benefits

    Environmental, social, economical co-benefits

  • Have potential

    Projects must have significant future scaling potential